1. Field of the Invention
This invention relates to a system comprising a laser diode and means for stabilizing the wavelength of a primary beam of light emitted by said laser diode, wherein said means comprise a controller for controlling the current supplied to the laser diode under the control of sensor means, which are responsive to the instantaneous ambient conditions and operating parameters, a beam splitter, which is disposed in the path of said primary beam and arranged to divert from said primary beam a partial beam to measuring means, which comprise photodetector means, which are illuminated by said partial beam via optical means with an intensity that varies in dependence on the wavelength of said primary beam, which photodetector means are operable to deliver output signals, which are optionally converted and are delivered as control signals to said controller.
This invention relates also to a laser interferometer which comprises a system as defined in the preceding paragraph, a second beam splitter for splitting said primary beam into a reference beam directed to a stationary reflector and a measuring beam, which is directed to a movable reflector that is adapted to be connected to a movable object, and reflecting means for diverting the reflected beams to photodetector means, which are connected to counters for detecting the fluctuations of the brightness resulting from the interference of the reflected beams during a movement of the movable reflector.
2. Description of the Prior Art
For numerous applications it is desired to stabilize the wavelength of a light bundle. One of such applications is the transmission of information in an optical waveguide. But the problems involved will subsequently mainly be discussed with reference to an interferometer.
Laser diodes have previously been used in laser interferometers only in special cases. DE-A-39 11 473 discloses stabilizing means of the kind described hereinbefore in conjunction with the laser diode of a laser interferometer. In that case the light emitted by the laser diode is split into a plurality of partial beams, the length of which is transmitted by respective monomode optical fibers. One of said fibers leads to a lens, from which the measuring light beam is directed along the measuring path. A second fiber transmits reference light to a receiver, and the measuring light beam is diverted by an adjustable measuring prism and is transmitted by a lens and the light of the measuring beam is subsequently transmitted by a further fiber to said receiver and in said receiver is caused to interact with said reference beam with interference.
For stabilizing the wavelength, light of two further beams is diverted and is transmitted by optical fibers of equal length to an additional interference generator to interact therein with interference. The exit openings of said optical fibers are arranged one beside the other so that the projected light beams can interfere and in the photodiodes spaced from the exit openings generate electric signals having an intensity that varies in dependence on the wavelength. A constant distance between the light exit openings and the photodetectors must be maintained for the measurement and for the stabilization of the wavelength.
From DE-A-39 30 273 it is known to use in a laser interferometer a solid-state laser, which is stimulated by at least one laser diode and converts the light emitted by the laser diode in the infrared range to light of a different wavelength. That solid-state laser consists, e.g., of a neodymium solid-state laser and can be used to stabilize the beam and the wavelength. To stabilize the wavelength a separate measuring interferometer is used, which receives a partial light beam, which has been diverted from the measuring beam. Any changes of the wavelength will result in the measuring interferometer in corresponding bright-dark changes, which can be utilized for an automatic control. That automatic control results either in a change of the current supplied to the stimulating laser diode or in a change of the control of the resonator surfaces of the solid-state laser.
As has been explained hereinbefore the accuracy of the measurement of length by an interferometer will depend on the fact that the wavelength of the laser light employed is known and is maintained constant. In single-frequency lasers the number of bright-dark changes is counted which in case of an adjustment of the movable reflector result from the interference between the reference beam and the measuring beam. In that case .DELTA.1=m.lambda./2, wherein .DELTA.1 is the change of the path length, m is the number of bright-dark changes, and .lambda. is the wavelength of the laser.
The intensity of the light which is received by the photodetectors varies according to a sine function. The resolution may be increased by the use of suitable circuits.
The wavelength depends on the frequency of the source of laser light and on the refractive index of the medium in which the light is transmitted. .lambda.=C/l and C=.sup.C o/.sub.n, where C is the velocity of light in a medium, C.sup.o is the velocity of light in a vacuum, l is the frequency of the source of laser light, and n is the refractive index. n will depend, inter alia, on the temperature, the humidity, the pressure of the air p, and the composition of the air. n can be calculated according to the so-called Edlen formula.
Commercially available interferometers comprise stable sources of laser light, such as Zeeman-stabilized He-Ne lasers. In the measurement, the air parameters are detected individually or by means of so-called wavelength trackers are detected jointly. If stabilized sources of laser light are employed, the frequency and, as a result, the wavelength cannot be changed and cannot be automatically controlled by usable control means but each result of the measurement must be corrected in computers with suitable correcting data.
Contrary to the stabilized He-Ne lasers, the frequency of laser diodes can often change in dependence on various operating parameters and such changes may often be undesirable. The frequency and wavelength will change particularly in dependence on temperature and on the current supplied to the laser diode. For instance, for a laser diode which is commercially available under the designation "HL7801 E", .DELTA..lambda./.DELTA.T=0.06 nm/.degree. C. and .DELTA..lambda./.DELTA.I=0.006 nm/mA.
The frequency may also change in dependence on the ageing of the laser diode. A use of the means known in conjunction with stabilized lasers for a correction of the result of measurement would be too expensive and would not give sufficiently accurate results. For this reason it has been attempted in the operation of laser diodes to stabilize the wavelength by a control of the current supplied to the laser diode. In that case the ambient conditions and operating parameters are detected by separate sensor means and a feedback loop for controlling the current supplied to the laser diode is operated in response to said sensor means.